Torque-limiting devices (TLDs) are often installed in vehicles and other equipment to prevent drive shafts from transmitting damaging torque to components connected to the drive shafts such as differentials and transmissions. These devices also prevent torque from damaging the drive shaft itself. In practice, a TLD is installed between two segments of the drive shaft, each segment being connected to one of the two flanges of the TLD. The two flanges of the TLD are rotatable with respect to each other, unless coupled by at least one mechanical fuse. When coupled by a mechanical fuse(s), the two flanges of the TLD rotate in unison, thereby transferring torque from a driving shaft segment to a driven shaft segment. The mechanical fuse fractures when a torque threshold is exceeded, thereby decoupling the two flanges and drive shaft segments connected thereto. Because the two flanges are no longer coupled they rotate relative to each other, preventing excessive torque transfer to the drive shaft segments and components connected thereto.
While torque-limiting devices as described above are effective in certain situations, they are not designed to accommodate other important service conditions, such as extended use of the TLD without a mechanical fuse in place or in drive trains subject to large radial forces. For example, in some applications the drive shaft itself may be heavy, or the drive shaft may be transferring large amounts of torque which can induce significant radial loads due to shaft bending moment. The mass of the drive shaft and/or the magnitude of the torque-induced bending moment then exerts a force having a component perpendicular to the rotational axis of the TLD, i.e., a radial force. In this situation, the sustained radial force to the TLD can cause the bearings in the TLD to fail. A similar situation occurs when the vehicle is traveling over rough terrain; the impact of the vehicle moving over the rough terrain can result in the application of shocks to the drive shaft coupled to the TLD that exert a force having a radial force component. Some designs have attempted to solve these deficiencies by having a protruding cylinder on one flange centered on the axis of rotation that mates with a confronting recessed cylinder on the opposing flange. While this arrangement can provide added resistance to radial forces, the two flanges can rust together thereby increasing the maximum torque threshold and risking damage to the drive train and components connected thereto. These problems are compounded when the torque-limiting device is subjected to a high rate of rotation.
Torque-limiting devices are generally not designed to accommodate the important service condition of a high rate of rotation, commonly measured in revolutions per minute (RPM). At high RPM, particularly over periods longer than approximately 5-60 minutes, depending upon the design of the TLD, the bearings can over-heat, degrading the bearing lubricant, and ultimately leading to bearing, and TLD, failure. The combination of a radial force exerted on the torque-limiting device during use at high RPM can accelerate bearing and TLD failure.